This invention relates to the measurement of the density of fluids.
There is an established technique known as vibration densitrometry which involves--in one form--filling a resiliently supported vessel with sample fluid and electromechanically exciting the vessel to a vibrational resonance. The resonant frequency can be determined precisely and is indicative of the mass contained in the known volume of the vessel and thereby of the fluid density. Reference is directed, by way of example, to U.S. Pat. No. 3,523,446.
Various shapes of vessel have been put forward as advantageous, including a sphere, a cylinder and a U-tube. If the apparatus is to have the sensitivity necessary to detect very slight changes in fluid density, a significant body of fluid must be contained within the vessel. However, with a sphere or cylinder of relatively large dimensions, the difficulty is encountered that the time of travel of a pressure wave through the contained fluid approaches the period of vibrational movement of the vessel. Under those circumstances it is no longer accurate to treat the vessel and fluid as together defining a rigid body moving in simple harmonic motion. Effectively, the central volume of the fluid plays less than its expected part in damping the motion of the vessel so that an erroneous density value is produced. This error becomes more significant with fluids of higher viscosity.
A further difficulty that is particularly acute with, for example, a vessel in the shape of a U-tube, is that with increasing dimensions the deviations from simple harmonic motion become more marked leading again to erroneous density values. In addition, there is an evident disadvantage in increasing the overall size of the measurement apparatus, particularly if it is the intention to mount the apparatus within process plant to provide on-line monitoring of density.
It has been proposed to deal with these problems by employing as the vessel an elongate tube shaped as a helical spring. Reference is directed, for example, to USSR Inventor's Certificate No. 711432 and USSR Patent No. 1062562. In each case the disclosed apparatus takes the form of a helical sample tube fixed at its ends and carrying at the mid length a pole piece driven by an electromagnet. Torsional waves are set up in the helical tube at a resonant frequency which is influenced by both the density and viscosity of the sample fluid contained within the tube. The resonant frequency is measured in a suitable manner.
There is a difficulty with this apparatus that the measurement it produces is both density and viscosity dependent. If viscosity remains constant, a separate measurement will enable a value to be determined for density, but this is troublesome and can slow down the procedure to a point where on-line determination becomes impractical. If the viscosity is not constant, the usefulness of the apparatus is in real doubt.
It is an object of this invention to provide improved apparatus for use in the measurement of fluid density which goes some way to overcoming the above disadvantages.